1. Field of the Invention
This invention relates liquid filters and, more particularly, to waste water filters which are periodically cleansed by forcing water through the filter in a reverse direction.
2. Description of the Prior Art
In the treatment of water supplies for potable, industrial and other uses, the water is commonly preconditioned by the addition of chemicals and then passed through settling basins or other means of pre-separation before filtering, or, in other designs, chemical treatment alone may be employed to precondition the water prior to passing it through filters for removal of the undesirable constituents.
Filters commonly used may employ only a simple media, with the bed being composed of multiple layers of media of different sized gradations, or they may be composed of several materials of varying sized gradations, placement and specific gravity. Regardless of composition, the filter bed is periodically cleansed of entrapped material by means of reversing the normal filter flow. This cleansing procedure is commonly known as the "backwash cycle." Backwashing is usually performed by forcing the backwash water in the opposite direction of the normal filter flow at rates which are sufficient to expand the filter media and wash the entrapped material out of the bed by means of a collection system, such as launderers or wash troughs. One of the problems with the prior art devices is the difficulty in reducing the quantity of suspended matter before filtering in order to retard clogging of the filter bed. An increase in the quantity of suspended matter quickly reduces the length of filter cycle relative to the backwash cycle so that much of the effluent flowing from the system is returned to the system as backwash waste. As a result a relatively small portion of the effluent remains after the backwash cycle, and the system is relatively inefficient.
Recent art has shown the effectiveness of the flotation process in reducing the quantity of suspended matter, much of which tends to float rather than to settle. Prior art devices generally used various clarification steps involving settlement of the suspended matter, but such clarification was both costly and only partly effective.
Using flotation ahead of the filtration step has until recently required a separate unit. In any case, successful flotation requires the release of dissolved gases.
In order to release the dissolved gases from the sewage in the optimum manner, it is necessary to pass a portion or all of the flow under pressure through a back pressure or throttling valve located at the inlet to the flotation section. It has often been very difficult to do this because of the wide variety of foreign matter frequently found in sewage which has a tendency to obstruct the free flow of water through the valves.